For inspecting the surface of micro-structured samples, such as wafers, masks or micro-structured devices on a substrate, optical apparatus are particularly suitable. For example as known from EP 455 857 the inspection of the surface can be carried out by evaluating beams retro-reflected from the surface of the wafer.
Optical apparatus are also known in which various structures on the surface of the sample of a wafer can be recognized by means of image detection. Herein the sample is usually illuminated in the bright field and sampled with a camera, such as a matrix or linear array camera.
The inspection of the surface of a wafer is also known from U.S. Pat. No. 6,587,193, wherein an illumination is chosen which samples the wafer in the form of a line. The illumination line is scanned across the surface of the wafer in such a way that a two-dimensional image can be created.
From US 2003/0202178 A1, furthermore a method and an apparatus for inspecting a wafer are known. Herein an illumination is radiated onto the wafer in such a way that it is incident on an edge of the wafer. The edge of the wafer can thus be detected and further processed by an image processing unit. Defects on the wafer can be detected by comparing the obtained edge image with a prestored reference image.
The prior art systems for inspecting wafers are almost exclusively designed for the incident-light inspection in the visual or UV ranges, and are generally not or not well suited for the inspection of micro-structured samples, such as encapsulated or embedded objects or wafers structured on both sides, and stacked structures of a plurality of wafers.